This invention relates in general to vehicle hydraulic systems and, in particular, to a hydraulic system for use in a vehicle such as earthmoving or construction machinery wherein the hydraulic system is utilized for operating the vehicle and actuating various auxiliary equipment or implements.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a vehicle hydraulic system wherein a primary pump, driven by the vehicle engine or torque converter, provides a source of pressurized hydraulic fluid to a steering control valve which is utilized to effect steering of the vehicle. A ground-drive auxiliary pump, which is actuated by movement of the vehicle in either a forward or reverse direction, provides a back-up system for the primary pump to insure a source of pressurized hydraulic fluid, at all times the vehicle is in motion.
In the operation of earthmoving and construction machinery, due to the size and weight of the vehicle and the function it must perform, hydraulics are utilized to control operation of the vehicle such as steering and braking, as well as to power the various auxiliary equipment or implements used with such vehicles. Such hydraulic systems are utilized for convenience, control of operations, and the safety of the machine operator and those working in close proximity to the machine. Power steering and braking, as well as the operation of the various hydraulic powered implements, require a source a pressurized hydraulic fluid to perform these functions, which is generally provided by a pump driven by the vehicle engine or torque converter. However, if the vehicle engine or torque converter stops while the vehicle is in motion, or if the vehicle is moved without power, no pressurized hydraulic fluid would be provided through such a system. Therefore, an auxiliary hydraulic system must be provided to insure that the vehicle may be safely steered and brought to stop regardless of whether or not it is operating under its own power.
In order to eliminate a potential safety hazard which might result if the primary hydraulic fluid pumping system is disabled or is not operating, an auxiliary or emergency hydraulic pumping system is incorporated in such vehicles. While such a system may be electrically driven, such as being powered by the vehicle batteries, a ground-drive auxiliary pump is frequently utilized. Such ground-drive pumps are powered by the physical movement of the vehicle in either a forward or reverse direction, and thereby provide a source of pressurized hydraulic fluid at all times when the vehicle is in motion. Since these pumps are in continuous operation while the vehicle is moving, the pump recirculates the hydraulic fluid into a sump or reservoir until such time as needed for operation. In the event that the flow of hydraulic fluid from the primary pump system is not sufficient, then the ground-drive pump is automatically available for powering the vehicle's hydraulic systems.
During normal operation, when the ground-drive pump is recirculating hydraulic fluid through the sump, it is desirable that any hydraulic fluid output from the primary pump which is not required by the vehicle steering system, be available for use by the auxiliary system to power the vehicle implements. While the auxiliary system generally has an implement pump to provide a source of pressurized hydraulic fluid for the auxiliary equipment or vehicle implements, the combining of the unused output from the steering pump with the implement pump output makes for better and more efficient operation. However, the steering pump's primary function is to provide a source of pressurized hydraulic fluid for the steering system. Therefore, a priority valve is used to couple the primary steering pump to the auxiliary system to maintain an adequate supply of hydraulic fluid in the steering system at all times, with the excess pressurized hydraulic fluid being available for use by the auxiliary implement system. The priority valve meters the output of pressurized hydraulic fluid from the primary steering pump to the steering control valve to maintain an adequate supply of hydraulic fluid for use in the steering system of the vehicle. The pressurized hydraulic fluid not needed to maintain this supply to the steering system is available for use with the auxiliary or implement equipment of the vehicle.
While such metering priority valve systems, and emergency steering systems, have been used in separate hydraulic systems, the present invention couples two priority valve controlled systems in a manner to permit different actuating pressure differentials, used to control the application of a ground-drive pump system to a vehicle steering system and the diversion of primary pump hydraulic fluid fluid flow from the steering system to an auxiliary or implement system, to be simultaneously controlled from a single pressure differential establishing source.